Effects of Conditioning and Digestion on Sludge Floc Structure

• Main Authors: Ching-Ping Chu, 朱敬平
• Other Authors: Duu-Jong Lee
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/86408669684669992461

博士 === 國立臺灣大學 === 化學工程學研究所 === 91 === Floc structure has been reported to be an essential factor in determining the treatment performance of the sludge. However, because most studies focused on the measurement of floc size and shape, the information on the complex geometrical morphology and microbiological ecology inside flocs has not been comprehensively discussed. A brief review in Chapter 1 introduced the recent research on the properties of flocs, focusing in particular on structural analysis. In Chapter 2, we evaluated the feasibility of several microscopic and tomographic techniques on investigating the sludge floc structure. The results showed that microtome slicing, confocal laser scanning microscopy (CLSM), free settling, and small-angle light scattering (SALS) were suitable for analyzing the pore size distribution and fractal dimensions of sludges subjected to various treatments. An image-processing algorithm, including defining the region of interest and the determination of bilevel thresholding value, was proposed for obtaining the geometric parameters from the microtome-sliced images and CLSM images. In Chapter 3, porous configuration of sludge flocs and filter cakes subjected to various treatments was examined using the techniques described in Chapter 2 to determine geometric parameters, such as porosity and pore size. The effects of conditioning, anaerobic digestion, and horizontal electroosmotic dewatering on the structure of sludge were reported. A possible correlation between structural information and process performance, such as dewaterability and digestibility, was also discussed. For sludge conditioning, cationic polyelectrolyte flocculation links the original sludge flocs into a large networked structure with highly non-uniform mass distribution. Freeze/thaw conditioning markedly compresses the flocs due to the gross migration. Saline curing causes the osmotic dehydration of the biomass granules. Notably, the free settling fractal dimension (DF) correlates with the porosity and pore size of flocs. The scattering fractal dimension (DS), however, exhibits no simple relationship with any geometric parameter. For sludge digestion, a correlation of sludge digestibility and floc structure exists. Both pre-hydrolysis and digestion cause the collapse of the highly porous configuration of flocs. The free settling fractal dimension and the geometric parameters pertaining to the large scale correlate with the sludge digestibility. However, the corresponding parameters of the small scale (in the scattering aggregates) barely reflect the difference of digestibility among the sludge samples. In the last section of Chapter 3, sludge cakes sampled at various positions and times in an electroosmosis-dewatering chamber were analyzed. The flocs accumulated at the anode due to the electrostatic attraction. The porosity continued to decrease as most of the biomass coagulated into large clusters. During the constant-rate period of moisture removal, the morphological differences between the cakes at the two electrodes became less significant. During the falling-rate period, a relatively large difference in cake porosity appeared between the electrodes, involving difference in pore size, porosity, Sierpinski fractal dimension (DSC) and pore boundary fractal dimensions (DB). In Chapter 4, the flow field in one sludge floc was simulated to calculate the pressure gradient and mass flow rate and estimate the floc permeability. Three-dimensional modeling software (Amira 3.0) was used to construct the meshes of the porous configuration of a sludge floc from the CLSM series images. Morphological parameters such as volume, surface area, box-counting fractal dimension and tortuosity were estimated using the model. The permeability KDL of original flocs is estimated as 8*10-12 m2. Flocculated flocs have higher KDL due to the large pore size, while the values of KDL of the freeze/thawed flocs decreased. The calculated results indicate that a few large pores in the floc apparently determine the magnitude of permeability. The fractal dimension and compactness, however, are not correlated with the permeability of the flocs. The commonly adopted permeability model, assuming a homogeneous interior, may be misused to estimate the permeability of a sludge floc with highly inhomogeneous biomass distribution.